Coated mold and method of preparing same



Patented Mar. 3, 1931 UNITED STATES PATENT OFFICE NELsoN .1. QUINN, orTOLEDO, OHIO, AssIeNon 'ro THE METAL CASTINGS nonnme CORZPOBATIOTT, OFCLEVELAND, OHIO, A CORPORATION OF DELAWARE COATED MOLD AND METHOD OFPREPARING SAME No Drawing. Application filed July 28, 1927, Serial No.209,159. Renewed liay 17, 1980.

order to improve the condition of the castings secured therefrom, and inorder, in fact, to secure castings which are acceptable and usableWithout any further treatment, such for example as annealing, which isthe case with all of the coatings now in use. Various methods of coatingand treating surfaces of permanent molds have been attempted, but nonehave been found successful and none have gone into general use, exceptin certain cases where all of the castings produced in such molds areannealed in order to soften them and remove the inevitable hard spotswhich'appear in various points in the casting.

The present invention relates to a mold provided with a coating and to amethod of preparing such coating which is of such a character that itproduces uniformly soft, finegrained castings which may be used withoutfurther treatment of any character, and which are equal or superior tocastings formed -in sand molds.

To the accomplishment of the foregoing and related ends, said'invention,then, consists of the means hereinafter fully described and particularlypointed out in the claims; the following description setting forth indetail one method and one product exemplifying my invention, suchdisclosed procedure and product constituting, however, but one ofvarious applications of the principle of my invention. I

Refractory materials, either alone or combined with various binders orcarbonaceous materials, have been used in the treatment of metal molds,but without any impression upon the general commercial article, and sofar as I am aware no such processes for coating are today in any generaluse, or have been found capable of producing uniformly soft,fine-grained and superior castings. I have found, however, that acoating of suitable refractory materials may. be applied to the matrixsurfaces of a metal mold to produce a relatively thin fixed coatingthereon which i is capable of producing satisfactory soft castlngsprovided the coating possesses certain characteristics as topermeability or porosity and capacity to withstand both the physicalwashof the molten metal cast thereagainst and the high, temperature of suchmetal, together with the proper insulating properties to prevent the toorapid transmission of heat through the coating to the metal mold,although the latter property is to some extent identical with' thatinitially named as permeability or porosity. The coating whichI havefound to be successful is formed with a multiplicity of cells orpassages therein, the porosity thus determined being obviousl variable,depending upon the nature 0 the article which is being formed andincreasing from a given minimum porosity suitable for the castmg ofarticles having thick sections to a maximum porosity which is essentialfor articles having very thin sections, or so formed that they cool orpermit the flow of heat rapidly.

The present invention is based upon the observation, which is of courseknown to the entire foundry trade, that sand molds are suc cessfulbecause of the permeabilityor porosity of the sand, and more and moreattention is being given to the maintenance of the permeability of sandwithin close limits when used in sand molds. Starting with thisobservation I have found materials which lend themselves particularlywell to the formation of coatings of high porosity and to methods ofapplying these materials to the matrix surfaces of metal molds, in sucha way as to produce a resulting coating of high porosity. These coatingsI have found satisfactory for producing uniformly 'soft castings andthis is universally regarded as'the one and only test of the success ofa coating for a metal mold. For obvious reasons the coating aplied to ametal mold must be relatively thin and the high conductivit of the'molditself compared with the con uctivity of a sand mold must be preventedfrom conducting away the heat of the cast metal with too great rapidity,and this result can be effected, so far as I, know, only by theinterposition .are, first, the porosity, for the reason stated,

then, equally important, a suitable resistance to the temperature of themolten metal cast thereagainst, and suficient physical strength towithstand the wash of metal as it is poured against the coating.Obviously the degree in which the above properties should be present inthe coating will vary with the type of article which is being formed.Thus castings in which large masses of metal, as for example, as atypical illustration, an iron ball, requires less porosity in thecoating since the total heat units in the molten metal prevent,

to a large extent, any sudden chilling with resultant hard spotsadjacent the exterior of the ball, and for such articles a coatinghaving a considerably less porosity can be used than that which isrequired for articles having thin sections, such, for example, as asmall piston, in which the section of the wall or skirt of the piston isrelatively'thin, and in which there is a very much greater danger ofchilling and hard spots. Similarly, the temperature of the metal whichis being cast will permit a selection from among the materials whichhave the other necessary properties, certain materials which areresistant to the temperatures at which alloyed metals are cast beingentirely insufficient for withstanding temperatures of steel or similarmetal. In each case it is possible for a certain selection of materials,to be used and I will therefore describe for convenience variousmaterials and methods which may be employed under varying conditions.

The method of treating the mold, that is,

of applying the coating to the matrix surface,

which forms a part of the present invention,

' consists, briefly stated, in suspending in a suitable liquid, such asWater, a quantity of finely divided refractory material, and thenspraying this emulsion against the matrix face of the metal mold, whilealso applying to the deposited emulsion sufficient heat to evaporate theliquid at a rate, and in a way, which permits the suspended solid matterto deposit and build up on the mold surface in a cellular or porouslayer. One method of securing this desired rate of evaporation toproduce a coating of suitable porosity to effect the results desired isto maintain the metal mold, during the spraying of the liquid andrefractory material, at a temperature above the boiling point of theliquid. In'this way heat is applied to the la er of liquid and solidmaterial from the sur ace of the mold. causing an evaporation of theparticles of mouse liquid in contact with the surface. These particleswhen evaporating bubble through the liquid coating above them, producinga cellular or porous condition of the refractory deposit which isdesired. At the same time heat must not be applied to the liquid coatingon the mold surface too fast or at such a rate that it will evaporatethe liquid before depositing, as in this case the solid deposit whichwill remain will not be cellular, but will be substantially solid andwillhave no materially better insulating properties than the materialitself, while it is an essential feature of my process and an essentialto the final mold structure to have an increased insulat ing value inthe coating applied to the mold in order to prevent the rapidtransferrence of heat from the molten metal to the mold it self, and inthis way to prevent the bad effects on the metal structure which alwaysfollow too rapid chilling of any parts of the casting.

In the continuous production of castings it may be necessary to cool themold between castings in order to prevent the temperature of the moldrising to such a point that it will too rapidly evaporate the coatingwhen sprayed on, or rather actually prevent the depositing of theliquid, and this may be done by any suitable means, such as an air blastdirected against the mold or otherwise.

The molds may be made of various metals or alloys of metals, providedthe particular metal used has a melting point which is high enough towithstand the heat of the metal being cast, and for ordinary purposes Iuse common cast iron molds which will handle temperatures in the moltenmetal of 3000 If. or over, it being understood of course that therefractory coatings protects the mold so that molten metals withtemperatures considerably in excess of the melting point of the metal ofthe mold may be applied.

Various refractorymaterials may be employed, all of which should be in afinely ground condition and capable of being readily suspended in aliquid vehicle for convenient spraying. Any suitable liquid may be used,although on account of its low cost and low boiling point I preferwater, to which may be added if desired a small amount of viscousmaterial or binder, such as syrup, molasses, glucose or similar materialin order to increase the adherence of the matrix coating to the surfaceof the mold and to prevent the rapid scaling off of this coating whenthe castings are withdrawn, necessitating a second application of thecoating, where otherwise several castings might be made before the moldsurface would necd to he recoated.

Of the various materials which may be used extremely porous coatings canbe formed of magnesium oxide, chalk, talc. diatomaceous earths, othermaterials high is required.

areaaae in magnesium content and certain metallic oxides and silicates,all of which should be.

finely divided and capable of remaining in suspension in the liquidused, and particularly in water. Less porous coatings, but coatingswhich are entirely satisfactory for castings having thick sections canbe made from various refractory clays, carbon in the form of graphite,and finely divided carbide. lit will be understood of course that all ofthe materials which may be used must have sufficient structural strengthto withstand the wash of the'molten metal thereagainst, and sufficientrefractoriness to withstand unchanged the temperature of the moltenmetal. By testing the porosity of coatings which have been foundsatisfactory for the production of thin sectioned castings and also forcastings having thick sections i have determined that the porosity musthe between 22% and 86%, but it is obviously impossible to define theexact porosity for all conditions as this quality varies directly withthe temperature of the metal being cast, the section of the metal beingcast, and possibly also with the conductivity of the metalrnold itself,governedby the thickness and the heat of the mold. For the production ofautomobile pistons of some three inches in diam eter and having arelatively thin section in the skirt l have found that a porosity lyingbetween 50% and 86% is required. The porosity was determined by firstmeasuring the volume of a portion of a coating applied to a mold, thencompressing the material comprising the coating under a pressure of 2000pounds per square inch, measuring the new volume and taking the ratiobetween the -initial and final volumes.

The mixture of liquid and suspended refractory material may be sprayedonto the tation of the mixture during the spraying process. lln applyingthe spray to the mold the point of issuance of the material from thespray nozzle should be brought sufficiently close to the face of themold so that the mixture will spread rapidly over the mold and build upa layer of coating of sufficient thickness to force the articles ofliquid next to the face of the mol which are first evaporated, to bubblethrough the coating in order to produce the cellular porous structurethat causes the mixture to carry an excess of air which assists inbuilding up the porous structure.

When'par-ticularly smooth castings are "desired the treatment of themold must be completed by directing the spray at the molten surface froma greater distance than when first applied. and either the same or adifferent mixture may be employed at this stage. By thus withdrawing thepoint of The operation of spraying,

issuance of the spray from the matrix face of the mold the deposit ofrefractory material will be very much thinner over the surface of themold, and the liquid will evapoproducing rate practicallyinstantaneouslg,i

e' thickness a surface layer of no apprecia which is fine-grained andacts to close the cells or pores in the coating proper.

l have secured very satisfactory results and perfectl uniform and finelygrained castings wit the use of molds treated in accordancev'vith thepresent invention, and castings in which a metal structure of veryclosely knit formation with an even distribution of carbon is secured.As an illustration,

l have made castings consisting of so-called open skirt automobilepistons which are extremely difiicult articles to produce, since theyhave a heavy section in the bosses, a second heavy section in the head,and an extremely thin section, as thin as in some cases, in the skirt. lhave produced such castings by the use of my improved molds which have auniform metal structure notwithstanding the great dilference in thethickness of this structure at various points,

and in which the metal is uniformly soft throughout the entire casting.Such a structure has generally been considered as impossible tosuccessfully make in metal molds.

Other forms may be employed embodying the features of my inventioninstead of the one herein explained, change being made in the form orconstruction, provided the ele-.

ments stated by any of the following claims or the equivalent of suchstated elements be employed, whether produced by my preferred method orby others embodying steps equivalent to those stated in the followingclaims.

I therefore particularly point out and distinctly claim as myinvention 1. In a method of applying a coating of a liquid containingrefractory material onto the matrix surface of a heated metal mold, the

steps which consist in sprayin said liquid by pressure from a spray nozze when held in close proximity to the surface to build up a relativelythick coating rapidly, and then removing said spray nozzle to a greaterdistance from said surface to produce a surface coating of .a differentcharacter. 7

2. A coating for a metal mold consisting of a refractory material havinga multiplicity of air cells therein acting to retard the transfer ofheat through said coating and having a smooth coating pf the samerefractory material of slight thickness overlying one surace of the saidfirst-named coating and int'egrally joined thereto";

3. A. coating for a metal mold consisting of a magnesium containingcompound havin suflicient strength to withstand the wash an d pressureof molten metal thereagainst and a porosity of not less than 25% normore than 86%.

4. A coating for a metal mold consisting of a magnesium containingcompound hav- 5 ing a porosity of not less than 50 per cent.,

nor more than 86 per cent.

.5. A coating for a metal mold consisting of a refractory materialhaving a lightness approximating that of magnesium oxide and having aporosity of not less than 50% and not more than 86%.

6. A coating for a metal mold consisting of a refractory material havinga lightness approximating that of magnesium oxide and 5 a porosity ofnot less than and not more than 86%. Y

7. A coating for a metal mold consisting of magnesium oxide of a highdegree of porosity..

2o 8. A coating for a metal mold for the production of uniformly softmachinable castings consisting of a refractory material havingsufficient strength to withstand the-Wash of the molten metalthereagainst and a po- 25 rosity of between 40% and 86%.

9. A coating for a metal mold consisting of a refractory material havinga multiplicity of air cells therein acting to retard the transfer ofheat through said coating and having a fine grained coating of lesserporosity of the same refractory material of slight thickness overlyingone surface of said first named coating and integrally joined thereto.

10. A permanent mold for the production of uniformly soft machinablecastings having a surface layer of refractory. material of a porosity ofnot less than nor more than 40 11. A coat-ing for a metal moldconsisting of a refractory material sprayed onto a heated mold surface,said material initially having properties in a loose, finely dividedcondition, of lightness and suspensibility in water approximating thoseof magnesium oxide, and said ,coating having a porosity of from 40% to86% and other properties approximating those of a coating of magnesiumoxide similarly applied.

Signed by me, this 22nd day of July, 1927.

NELSON J. QUINN.

